Because of their relatively high energy density and their current availability, fossil fuels, such as coal, are currently used to supply most of the world's energy requirements. Unfortunately, use of such fuels is thought to generate a substantial portion of the greenhouse gas emissions. Thus, as global demand for energy and awareness of possible environmental damage caused by the use of fossil-fuel energy sources increase, it becomes increasingly desirable to use such energy sources more efficiently, while mitigating any negative environmental effects.
One technique that has been developed for more efficiently using coal and mitigating deleterious environmental effects includes gasification of coal to produce substitute natural gas (SNG). Producing SNG from coal is desirable because the produced SNG can be used in existing natural gas infrastructure (e.g. pipelines, compressor stations, and distribution networks), in commercial applications where natural gas is a feedstock, in domestic applications where natural gas is used for heating and cooking, and in electric utility applications where natural gas is used as a fuel to produce electricity. Coal reserves are substantially greater and more accessible than natural gas supply, and SNG can provide an additional supply of natural gas as the supply of existing natural gas sources diminish. Producing SNG from coal also has the added advantages of providing stability to the supply and thus price of natural gas and of being a higher density, cleaner burning fuel, as compared to coal.
Techniques to gasify coal produce a gas called “syngas,” which is a low heating value gas composed of hydrogen and carbon monoxide, and which generally cannot be used as a substitute in natural gas applications, are also generally known. Syngas, also known as municipal gas, has been produced for 100 years in North America. The syngas production releases greenhouse emissions into the atmosphere, and syngas has a much lower heating value (BTU/scf) compared to SNG, which is composed primarily of methane (CH4). Although syngas can be converted into methane, the use of a catalyst is required, and hence a relatively expensive, two step process with low efficiency is required for such conversion.
Although some techniques for gasifying coal and the production of methane or SNG from coal are known, such techniques are relatively poor at capturing potential greenhouse gasses and may be relatively expensive. Accordingly, improved apparatus and techniques for producing SNG and electricity from coal are desired.